This invention relates to apparatus for measuring operating parameters of a high voltage power conductor. More particularly, it relates to radio transmitting sensors which are mounted on the transmission lines. The sensors normally derive their power through electromagnetic induction using the magnetic field generated as a result of current flowing through the transmission line conductor. Transmission line sensors of the above type are disclosed in the U.S. patent of Howard R. Stillwell and Roosevelt A. Fernandes entitled Transponder Unit for Measuring Temperature and Current on Live Transmission Lines, U.S. Pat. No. 4,384,289, issued May 17, 1983, which patent is incorporated herein by reference.
Various power line sensors have been disclosed in the prior art. For example, see U.S. Pat. Nos. 3,428,896; 3,633,191; 4,158,810 and 4,268,818. Sensors of this type and of the improved form disclosed in the above-identified Stillwell and Fernandes patent have been proposed for dynamic line rating of electrical power transmission lines. See for example, papers numbered 82 SM 377-0 and 82 SM 378-8 entitled DYNAMIC THERMAL LINE RATINGS, PART 1, DYNAMIC AMPACITY RATING ALGORITHM; and, DYNAMIC THERMAL LINE RATINGS, PART II, CONDUCTOR TEMPERATURE SENSOR AND LABORATORY FIELD TEST EVALUATION; papers presented at the Institute of Electrical and Electronic Engineers P.E.S. 1982 summer meeting. These papers are incorporated herein by reference.
The power line sensors available in the prior art measure quantities associated with operation of the overhead line such as voltage on the conductor, current flow in the conductor, conductor temperature and ambient temperature. The information gathered is transmitted to a local ground station. Data from various ground receivers is transmitted to a central control station where the information is analyzed. Based on this analysis, the entire power system is controlled with power being supplied to the various transmission lines in accordance with their measured parameters.
The compact toroidal-shaped sensors, disclosed in U.S. Pat. No. 4,384,289 and in the prior copending applications of Fernandes et al., contain the sensor electronics, power supply and transmitter circuits, and hardware for attaching the sensors to the transmission lines. These sensors are operable only when the line current of the monitored conductor is at or above the minimum value necessary to power the sensor electronics. It is often desirable, however, to permit sensor operation when line currents are below the threshold level, i.e. for monitoring very low current conditions or detection of energized conductors with zero current flow.
It has been suggested that a battery be added to the power supply to allow sensor operation when the line current is insufficient to power the sensor and transmitter circuits. However, this arrangement would require periodic removal and/or replacement of the battery for recharging, with back-up capability always being uncertain. The present invention avoids the disadvantages of the prior art by providing a reliable power back-up system.
In accordance with the present invention, a toroidal-shaped transmission line sensor module is provided with a battery back-up facility which provides power for the module during very low current line conditions. The sensor module power supply, which derives power from transmission line current, will power the sensor module electronics under normal operating conditions. The battery is maintained in a float mode which enables it to be recharged by the power supply if the charge level of the battery is depleted. A sensing circuit detects if the regulated voltage supplied by the power supply is less than the battery voltage. If this condition is detected, power to the sensor electronics will be supplied by the battery. If this condition persists and the battery charge falls below a certain level, the load is removed from the battery to prevent damage.
It is therefore an object of the invention to provide a transmission line sensor which can operate under the condition of low transmission line current.
Another object is to provide power back-up for the sensor when the transmission line current is below a threshold level.
A further object of the invention is to provide monitoring circuitry which switches between main power and back-up battery power as a function of the capacity of the line current to provide a regulated voltage through the power supply that is higher than the battery voltage.
Another object of the invention is to provide monitoring circuitry which inhibits data transmission when the battery charge is depleted to a certain level.
Still another object of the invention is to provide a power back-up system in which the back-up battery is constantly float charged when the power requirements of the sensor module are being met fully through electromagnetic induction from the line current.